In the past, liquid crystal display devices of the active matrix type (using Thin-Film Transistors (TFTs), etc.) or simple matrix type (Super-Twisted Nematic (STN), etc.) have been in use. It is well known that in all of these liquid crystal display devices, the light transmittance was dependent on the viewing angle, because the light passed through liquid crystals of different relative alignment states according to the angle from which the screen was viewed. In particular, such screens were difficult to see when viewed from the side. Accordingly, there has been much research seeking to improve these viewing angle characteristics.
FIG. 13 is a plan view showing the structure of one pixel in the active matrix substrate of the liquid crystal display element of Japanese Unexamined Patent Publication No. 5-273569/1993. As shown in FIG. 13, on the active matrix substrate, a Thin-Film Transistor 55 is provided adjacent to the intersection of a gate line 52 with a source line 53.
As shown in FIG. 14, the TFT 55 is made up of the following layered on a transparent insulating substrate 51a of glass or similar material: a gate electrode 56 which is connected to the gate line 52, a gate insulating film 57, and a semiconductor layer 58, which is made of amorphous silicon and which is layered on the gate electrode 56. On the semiconductor layer 58, in sections in order to partially cover the semiconductor layer 58, are n.sup.+ -Si layers 59, which serve as ohmic contact layers. On one n.sup.+ -Si layer 59 is a source electrode 60, which is connected to the source line 53, and on the other n.sup.+ -Si layer 59 is a drain electrode 61, which is connected to a pixel electrode 62.
The pixel electrode 62 is provided in the rectangular area bordered by the gate line 52 and the source line 53, and distributed on the pixel electrode 62 in islet form are transparent insulating films 63 made of SiN.sub.x, SiO.sub.2, or similar material. Each transparent insulating film 63 also serves as a protective film. The active matrix substrate is completed with the covering of TFT 55, pixel electrode 62, and transparent insulating films 63 with an alignment film (not shown).
On a transparent insulating substrate 51b, provided opposite the active matrix substrate, are layered a counter electrode 65 and an alignment film (not shown), in that order. The liquid crystal display element is completed by filling the space between the two substrates with a liquid crystal 66.
In a liquid crystal display device of this structure, at a', where the transparent insulating films 63 do not cover the pixel electrode 62, the image signal voltage is applied directly to the counter electrode 65, but at b', where the transparent insulating films 63 are provided, a divided capacitance voltage is applied, because the image signal voltage is applied through the serial capacitance of the capacitance of the liquid crystal 66 and the capacitance of the transparent insulating film 63.
In this way, two domains are created within one pixel, each of which applies a different voltage to the liquid crystal 66, resulting in a different light transmittance of the liquid crystal 66 within each domain. Accordingly, viewing angle characteristics when viewing the screen from the side can be improved. Further, by forming a tapered section c' around the edges of the transparent insulating films 63, a clear image without rough edges can be obtained.
However, a disadvantage of the conventional structure outlined above is that the inclusion of the transparent insulating films 63 on the pixel electrode 62 creates a plurality of areas that have uneven surfaces, resulting in disturbance of alignment and deterioration of display characteristics. The thicker transparent the insulating films 63 are made (in order to increase the difference between the voltage applied at area a' and that applied at area b'), the more likely the disturbance of alignment. Further, another problem with increasing the thickness of the transparent insulating films 63 is that gap control becomes difficult, because the cell gap at area a' is different from that at area b'.
In order to solve these problems, Japanese Unexamined Patent Publication No. 7-175037/1995, shown in FIG. 15, disclosed a liquid crystal display device which would prevent the deterioration of alignment by providing a thicker alignment film 67 on the active matrix substrate side, and by making the interface between the alignment film 67 and the liquid crystal 66 a flat surface.
However, in making the surface of the alignment film 67 flat in order to prevent the disturbance of alignment, a minimum film thickness of approximately 0.5 .mu.m (=500 nm) is necessary, but the applied voltage must be increased, leading to the problem of increased power consumption. Further, the polyimide generally used for the alignment film 67 is not completely colorless, thus decreasing the display quality.
In general, the active-matrix-type liquid crystal display devices have had a comparatively wide viewing angle with good display quality, but the simple-matrix-type liquid crystal display devices have had a narrower viewing angle. For this reason, users selected the type of liquid crystal display device according to their needs, using the active-matrix-type devices having wider viewing angles, for example, for regular use in the office and elsewhere, or for presentations, while using the simple-matrix-type devices having narrower viewing angles in situations calling for privacy, for example, when preparing documents aboard aircraft or elsewhere in public. However, since there were cases when the same liquid crystal display device was used both in the office and aboard aircraft and elsewhere, purchasing a device with a viewing angle suited for one type of use created inconveniences when using the device in other ways.
Japanese Unexamined Patent Publication No. 6-59287/1994, discloses a liquid crystal display device which can meet both types of needs with one device. This liquid crystal display device, as shown in FIG. 21, controls the viewing angle using a TN-type liquid crystal display panel 151 and, for viewing angle control, a guest-host-type liquid crystal panel 152. In concrete terms, when using the device in the office or for a presentation, no voltage is applied to the guest-host-type liquid crystal panel 152, thus scattering the light and enabling a wide viewing angle (see FIG. 21(a)); when using the device aboard an aircraft or where viewing of screen images by others is unwanted, a voltage is applied to the guest-host-type liquid crystal panel 152, allowing the light to be transmitted in one direction only, increasing the parallelism of the backlight, and thereby enabling a narrow viewing angle (see FIG. 21(b)).
However, since this liquid crystal display device uses two liquid crystal panels, the thickness and weight of the device as a whole is increased, as are costs. Further, power to drive the two liquid crystal panels is necessary, as well as power to ensure that the backlight is not dimmed due to passing through the two liquid crystal panels, resulting in the problem of increased power consumption. For these reasons, the liquid crystal display device discussed above could not be used for laptop-type personal computers or other portable information terminals.